Add a project_step_config sub

Which is the pc template function.

This subroutine does the same as project_config, except that it uses the
step defined in the options (if it's defined), and a clean $config->{run}.
input_files: use the right step

When no filename is defined for an input file that is a project, we get
it from the project's config. If pkg_type is defined, we should use it
as the step.